1. Field of the Invention
The present invention is related to an image forming apparatus and, more particularly, to an image forming apparatus which is capable of forming an image on the back of an image receiving medium of which a front surface has another image formed thereon.
2. Description of the Related Art
As an image forming apparatus, a copying machine equipped with an automatic duplex device is known. When forming images on both sides of a paper by this copying machine, an image is first formed on the surface of a paper supplied from a paper supply cassette, etc. by an image forming unit. Then, the paper on which front surface an image is formed is stacked in a temporary stacker of the automatic duplex device after passing through a fixing unit. The stacked paper is taken out of the temporary stacker and supplied to the image forming unit again in the inverted state and then, another image is formed on the back of the paper by the image forming unit.
In general, the automatic duplex device is provided with a pick up roller to pick up a paper from the temporary stacker, a feed roller to supply the picked up paper, and an aligning roller pair to align the leading edge of supplied paper and supply it to the image forming unit at a prescribed timing. A paper is stacked in the temporary stacker in the state with the image formed surface facing upward, and the pick up roller and the feed roller convey the paper by contacting its image formed surface.
When conveying a paper by having the feed roller contact its image formed surface as described above, the coefficient of friction between the feed roller and a paper changes corresponding to the density of an image formed on the paper surface. That is, if a fully black image in high image density or an image in high half tone was formed on a paper, a quantity of developer adhered to the paper is much more than an image in the low image density like an image having much white portion. So, the surface of a paper with a high image density has the small coefficient of friction and is in the slippery state.
However, on a conventional image forming apparatus, such paper supply conditions of a feed roller as a driving speed, a driving time, etc. are always set at constant levels regardless of the image density formed on a paper. Therefore, when a paper on which an image formed in the low image density and having the relatively high coefficient of friction is supplied by the feed roller, the amount of feeding operation of the paper to be supplied to the aligning roller becomes much more than a specified amount of feeding operation. In this case, the leading edges of the paper are deflected and a transfer void, paper jam, etc. tend to occur. On the contrary, when a paper with a high density image is formed and the relatively low coefficient of friction is supplied by the feed roller, the amount of feeding of the paper to be supplied to the aligning roller becomes less than a specified amount, causing an insufficient aligning. As a result, the slippage of the leading edges of paper, paper jamming, etc. tend to occur.